1. Field
Embodiments of the present invention relate to laser systems and, in particular, to temperature compensation in lasers systems and their associated control systems.
2. Discussion of Related Art
One link of an optical telecommunication system typically has a transmitter, an optical fiber, and a receiver. The transmitter has a light source, which converts an electrical signal into the optical beam and launches it into the optical fiber. There is information on a data stream in the electrical signal that is also modulated onto the optical beam. The optical fiber transports the optical beam to the receiver. The receiver converts the optical beam back into an electrical signal and recovers the information from the data stream. Laser systems, such as those that use distributed feedback (DFB) lasers, external cavity lasers (ECL), and vertical cavity surface emitting lasers (VCSELs), are common coherent light sources.
To ensure proper operation of any laser system, many of the parameters (e.g., power, channel, temperature) are controlled and monitored by control loops. One such control loop is an automatic power control loop, which is designed to maintain average optical power out of the laser constant, typically because as lasers age the power output at a given laser bias current decreases and as they change temperature, their slope efficiency changes, resulting in different amounts of light output for the same bias current.
A typical automatic power control loop includes a monitor photodiode positioned at the back facet of the laser to monitor the power output from the laser. The light in the optical beam emitted out of the back facet is either substantially the same as light in the optical beam emitted out of the front facet or has a known proportionality to light out of the front facet so that the monitor photodiode provides a good indication of the power in the optical beam light out of the front facet. The automatic power control loop adjusts the laser bias current in response to laser temperature changes and/or aging as sensed by the monitor photodiode.
The light emitted by the laser has what is sometimes called direct current (DC) components. The DC component of a high-speed laser is average optical power. Average optical power (AOP) is the average level of power in the optical beam over a time constant much longer than the period of one bit of data. However, optical receivers respond to signal swing rather than average optical power, which is not affected in a typical alternating current (AC) coupled laser system by adjusting the laser bias current in response to laser temperature changes and/or aging.